CONTRIBUTION OF VOLCANISM AND TECTONISM TO AXIAL AND FLANK MORPHOLOGYOF THE SOUTHERN EAST PACIFIC RISE, 17-DEGREES-10'-17-DEGREES-40'S, FROM A STUDY OF LAYER 2A GEOMETRY

Multichannel seismic images from the East Pacific Rise 17 degrees 10'- 17 degrees 40'S are used to study the geometry of seismic layer 2A in the near-axis region. Wave-equation datuming is applied to stacked com mon midpoint data to remove the distorting effects of seafloor topogra phy on the layer 2A event. Processed stacks are compared with seafloor morphology and tectonic fabric imaged in side-scan sonar data. Assumi ng layer 2A corresponds with the extrusive crust, these data are used to study the relationship between volcanism and tectonism in the accum ulation of the extrusive layer and to assess the contribution of extru sives to ridge-crest and ridge-flank morphology. We find variations in axial thickening of layer 2A which imply twofold variation in the wid th of the extrusive layer accumulation zone, as well as systematic cha nges in the pattern of accumulation of this layer. On the ridge flanks the layer 2A horizon mimics abyssal hill relief, consistent with a ho rst and graben origin for this topography. Short-wavelength variations in the thickness of layer 2A are superimposed on this relief, which w e attribute to volcanic modification of tectonic topography at the edg e of the neovolcanic zone. The integrated off-axis thickness of layer 2A is correlated with ridge cross-sectional area and indicates persist ent (100,000 years) spatial gradients in extrusive layer thickness. Sy stematic changes in the cross-axis shape of the ridge, observed over d istances of 5-10 km, cannot be attributed to the extrusive layer, and it appears that axial structure beneath the volcanics governs the cros s-axis morphology of the ridge.